1. Field of the Invention
The present invention relates to a color filter to be patterned on a cathode ray tube (CRT) so as to enhance the brightness and contrast characteristics of the CRT, and more particularly, to a suitable color filter solution for a color filter and a method for pattering a color filter therewith.
2. Description of the Prior Art
Typically, a cathode ray tube(CRT) is a device for forming an image by excitation light emission of a phosphor of red, green, and blue of a phosphor screen by landing an electron emitted from an electron gun and accelerated by high voltage on the phosphor.
Various characteristics are required to the phosphor screen of the CRT, but the most important characteristics are brightness and a contrast.
Generally, a dark-tint glass panel with 40-50% transmittance, a semi-tint glass panel with 50-60% transmittance and a clear panel with 80-90% transmittance which are classified in accordance with its light transmittance have been used to improve the contrast in the CRT.
However, there are a wide difference in the light-transmittance among the above panels and more particularly, the contrast and brightness characteristics are appeared contrary to each other. In the dark-tint glass panel, rays of the light incident thereon from the outside thereof are considerably absorbed by the dark-tint glass panel to improve contrast, but rays of the light exited by the phosphor are also absorbed to decrease brightness.
Conversely, in the clear panel, the rays of the light exited by the phosphor are mostly passed through the clear panel thereby to increase the brightness but the rays of the light from the outside are reflected to decrease the contrast.
Accordingly, the contrast and brightness are contrary to each other, so that it is very difficult to increase the both characteristics.
Applying of a pigment to the phosphor to absorb the rays of the light from the outside is also known. That is, a CRT, in which a layer including a opaque pigment of uniformly sized particles is provided between a face glass and phosphor screen, is disclosed. However, a part of the rays of the light exited by the phosphor is absorbed in the pigment, so that the 10-15% decrease of brightness is estimated.
Another type CRT is disclosed to solve the problem that the increasing of the contrast in the CRT carries the decreasing of the brightness. The CRT comprises a glass panel of which the inner side is provided with the patterned color filters of a green, a blue and a red, respectively, so that the rays of the light with the corresponding wavelength are transmitted and the rays of the light with other wavelength are filtered when the exited rays of the light are passed through the filter, thereby cause to improve the brightness, contrast, and color producing capacity in the CRT.
If the above technique is employed in the clear panel with a high transmitance of light, the contrast can be enhanced with the outside rays of the light absorbed in the color filter together with the enhancement of the brightness by the characteristics of the clear panel.
To obtain the above CRT, a black matrix should be deposited on the glass panel and the color filters are patterned correspondingly to the green, blue and red phosphor to be patterned, respectively followed by patterning the corresponding phosphor to be multi-layered.
However, such techniques would cause to decrease the production efficiency because it needs a number of processes that the respective color filters of three kinds have to be patterned by a separate process. Further a rate of loss of a slurry for the color filters is increased and the cost is risen together with a decrease of the yield because of the difficulties of the management of the processes.
Furthermore, in the case of a red filter, Fe.sub.2 O.sub.3 as a filter material, has a property absorbing the violet ray upon its exposure, and thus the patterning by a normal photoresist method is difficult. Thus, the red filter patterning should be processed with the both photoresist and etching methods, which cause increase the steps together with a pollutant discharge.
On the other hand, some conditions are required to apply color filter slurry on the black matrix. At first, a well-dispersed and stable color filter solution is required which has a good transmission characteristic and has no reactions with a photoresist (e.g., polyvinyl alcohol and sodium dichromate) added therein and with a black matrix upon applying.
In the typical dispersion method of color filter, a pigment of super fine particles, citric acid as a dispersion agent and a glycol solvent have been used.
However, such method provides another problem together with a light transmitrance. That is, when the polyvinyl alcohol is added, the color filter solution is gelatinized thereby causing the black matrix to be contaminated or to be fallen off. The color filter solution is so sensitive to pH. It is stable under pH 5.0 but when the pH becomes lower, the particles in the solution are cohered because the potential of zeta is lowered. It is thus difficult to obtain the desired transmittance characteristic of the color filter and a cloudy screen is obtained.
Accordingly, it has been required to provide a slurry preparing process and an easier red filter patterning process for obtaining a desired transmittance and exposing conditions of a color filter by means of a photoresist techniques with a slurry sensitive to light in the CRT with a color filter.